A pure amorphous form of rosuvastatin calcium substantially free from alkali metal impurities is disclosed. A process of preparing a pure amorphous form of rosuvastatin calcium is disclosed, which comprises hydrolysis of C1-C5 alkyl esters of rosuvastatin, preferably tert-butyl ester of rosuvastatin, with an organic nitrogen base, e.g. guanidines, amidines, amines and quaternary ammonium hydroxides, in the presence of water, optionally containing aprotic solvent, following the conversion of thus obtained rosuvastatin salt with a source of calcium to obtain rosuvastatin calcium, which is then isolated. An alternative process is disclosed, which comprises the conversion of numerous novel ammonium salts of rosuvastatin, preferably tert-octylammonium salt of rosuvastatin, with the source of calcium to desired commercial rosuvastatin calcium. Rosuvastatin calcium is an inhibitor of HMG CoA reductase, useful in the treatment of hyperlipidemia, hypercholesterolemia and atherosclerosis.